Clarity of glass trumps magnification. You're better off with a Kowa 554 (15-45x) than with a Vortex Diamondback (20-60x) due to the significantly better glass in the Kowa.
Those that often provide hard numbers (ex: consistently field judge a bull elk to the inch at 3 miles with hand-held 10x binoculars) are full of cow excrement. There are too many variables to put a hard number on maximum distance for any optic.
Here is a real life example of what I encountered while scouting for a Coues hunt (overlapping terrain for mule deer and Coues):
Glassed up a bedded deer with my SLC 15s less than 1200 yards away. Kept glassing and decided to use my spotter (STX 95) to get a closer look at that deer. Yep, looked like a mule deer doe. So I moved on which happen to cut the distance to the deer. So I looked at my SLCs and it still was a doe. Kept glassing for Coues, checked and the doe was still there. Took a look through my STX 95 and yep, it was a doe. That was until she turned her head and had a large rack (170 class).With the buck tucked tight up in a bush, I could not make out the rack until it moved. This is with alpha glass at under 1000 yards under a bright and clear afternoon (damn near ideal conditions).
Focus on getting the clearest glass you can in your budget even if it means taking a hit on magnification.